Solid polymer electrolyte fuel cell power plants are known in the prior art, and prototypes are even available from commercial sources, such as Ballard Power Systems, Inc. of Vancouver, Canada. These systems are serviceable, but are relatively complex. An example of a Ballard Power Systems polymer membrane power plant is shown in U.S. Pat. No. 5,360,679, granted Nov. 1, 1994.
In addition, known fuel cell constructions commonly include a proton exchange membrane disposed between respective cathode and anode plates. In general, the operation of a proton exchange membrane (PEM) fuel cell includes the supply of gaseous fuel and an oxidizing gas to the anode electrode plate and cathode electrode plate, respectively, and distributed as uniformly as possible over the active surfaces of the respective electrode plates, or, more specifically, the electrode plate surfaces facing the proton exchange membrane, each of which typically includes a catalyst layer therebetween. An electrochemical reaction takes place at and between the anode plate and cathode plate, with attendant formation of a product of the reaction between the fuel and oxygen, release of thermal energy, creation of an electrical potential difference between the electrode plates, and travel of electric charge carriers between the electrode plates, with the thus generated electric power usually constituting the useful output of the fuel cell.
One problem occurring in solid polymer fuel cells relates to the management of water, both coolant and product water, within the cells in the power plant. In a solid polymer membrane fuel cell power plant, product water is formed by an electrochemical reaction on the cathode side of the cells, specifically by the combination of hydrogen ions, electrons and oxygen molecules. The product water must be drawn away from the cathode side of the cells, and makeup water must be provided to the anode side of the cells in amounts which will prevent dryout of the proton exchange membrane, while avoiding flooding, of the cathode side of the electrolyte membrane.
Austrian Patent No. 389,020 describes a hydrogen ion-exchange membrane fuel cell stack which utilizes a fine pore water coolant plate assemblage to provide a passive coolant and water management control. The Austrian system utilizes a water-saturated fine pore plate assemblage between the cathode side of one cell and the anode side of the adjacent cell to both cool the cells and to prevent reactant crossover between adjacent cells. The fine pore plate assemblage is also used to move product water away from the cathode side of the ion-exchange membrane and into the coolant water stream; and to move coolant water toward the anode side of the ion-exchange membrane to prevent anode dryout. The preferred directional movement of the product and coolant water is accomplished by forming the water coolant plate assemblage in two parts, one part having a pore size which will ensure that product water formed on the cathode side will be wicked into the fine pore plate and moved by capillarity toward the water coolant passage network which is inside of the coolant plate assemblage. The coolant plate assemblage also includes a second plate which has a finer pore structure than the first plate, and which is operable to wick water out of the water coolant passages and move that water toward the anode by capillarity. The fine pore and finer pore plates in each assemblage are grooved to form the coolant passage network, and are disposed in face-to-face alignment between adjacent cells. The finer pore plate is thinner than the fine pore plate so as to position the water coolant passages in closer proximity with the anodes than with the cathodes. The aforesaid solution to water management and cell cooling in ion-exchange membrane fuel cell power plants is difficult to achieve due to the quality control requirements of the fine and finer pore plates, and is also expensive because the plate components are not uniformly produced.
In the fuel cell technology, the water transport plate is a porous structure filled with water. During fuel cell operation, the water transport plate supplies water locally to maintain humidification of a proton exchange membrane (PEM), removes product water formed at the cathode, removes by-product heat via a circulating coolant water stream, conducts electricity from cell to cell, provides a gas separator between adjacent cells and provides passages for conducting the reactants through the cell. The water transport plate supplies water to the fuel cell to replenish water which has been lost by evaporation therefrom. This system and operation thereof is described in U.S. Pat. No. 5,303,944 by Meyer, U.S. Pat. No. 5,700,595 by Reiser and U.S. Pat. No. 4,769,297 by Reiser, each incorporated herein by reference. Due to the constraints of the water transport plate formation process, these plates are costly to manufacture and possess limited strength.
For example, water transport plates can be formed in a dry-laid process where graphite powder and powdered phenolic resin are showered into a mold to form a layer. The layer is compacted to form a 0.100 inch thick layer which is heated until the phenolic resin melts and coats the graphite powder. The resin is then cured, thereby bonding the graphite powder in a composite. Although this is a common water transport plate formation process, the forming speed is slow and it is difficult to incorporate relatively long fibers which are desirable for water transport plate structural integrity. Longer fibers tend to become entangled in the dry-laid feeder, thereby forming fiber bundles in the finished composite. This fiber bundling, which corresponds to uneven fiber distribution, creates weak areas within the composite which are susceptible to structural failure. Composite structural integrity is maximized at fiber lengths greater than about 1.0 mm (about 0.040 inches) while the dry-laid process is limited to fiber lengths of about 0.51 mm (about 0.02 inches). Consequently, the tolerances in the specification for the water transport plate are small and the fabrication is difficult, resulting in many rejected parts.
In addition, the environmental and operational parameters of a water transport plate must be carefully balanced to obtain optimum performance of the overall fuel cell. For example, parameters of the water transport plate such as pore size, resistivity, particle size, resin content and yield strength, must be properly selected to obtain bubble pressure characteristics and water permeability which are acceptable for efficient operation of the fuel cell.
A major concern with PEM fuel cells is the water management with the cell. This is of particular concern when employing porous members such as the water transport plates discussed above. This porosity is needed to supply to and substantially uniformly distribute over the respective active surface the respective gaseous medium which is fed through respective channels provided in the anode water transport plate and the cathode water transport plate to the areas of the respective electrode plate that are spaced from the proton exchange membrane. Also, these porous structures are used to remove the reaction water from one of the active surfaces and supply of water to the other active surfaces to avoid drying out of the proton exchange membrane.
When porous water transport plates are employed in a PEM fuel cell, it is necessary to ensure that neither any liquid, such as product or coolant water in a PEM fuel cell, nor any gaseous media such as the fuel or oxidant, be able to flow out of the periphery or edge of the respective porous water transport plate. The escape of water through the periphery or edge of the water transport plates or migration of water proximal to the periphery or edge results in the loss of the respective media within the water transport at hand causing a decrease in fuel cell efficiency. Most importantly, preventing the escape of media through the periphery or edge of the water transport plate is critical to avoid the mixture of gaseous fuel with the oxidizing gas or ambient air which could be catastrophic.
Various attempts have been made in the prior art to avoid the escape of media from the cathode water transport plate and the anode transport plate in a PEM fuel cell. One such attempt is described in U.S. Pat. No. 5,523,175 by Beal, incorporated herein by reference. For example, the edge portions of the plates have been coated with a layer of various materials, such as polytetrafluoroethylene, to prevent the media from escaping. Also, densification of the edge regions of a water transport plate by impregnating the plate with a liquid substance which is later cured to leave behind a residue. This residue assists in preventing escape of fuel cell gaseous media, however, the gaseous media still is able to reach the edge of the plate thus causing undesirable leakage. One deficiency of the U.S. Pat. No. 5,523,175 is that the useable polymers are somewhat hydrophobic. This results in the interface between the polymer filled edge and water filled central region being hydrophobic which results in local leakage of the gaseous media. Liquidous cell electrolyte has been used in the prior art to provide capillary media within a fuel cell. However, such use of electrolyte is inappropriate in PEM type fuel cells where the electrolyte is solid where liquid water is the by-product of the electrochemical reaction of the fuel cell and where water transport plates become filled with water during operation of the fuel cell.
In view of the foregoing, an improved fuel cell is desired which includes improved edge seal characteristics for efficient fuel cell operation. It is also desirable for a PEM fuel cell to include an efficient and reliable structure for containing reactant gases without the need for additional gaskets or the impregnation of the edge portions of the water transport plates of the cell.